Recovering bad sealed lead acid batteries

I took a SLA battery, 12V 5A out of a portable amplifier earlier. Ordered a new one and sat the knackered one on the workbench. I've thrown so many of these things away and tried to recover them etc without any real success. So today's 'background' challege was set while I got on with other stuff.

The battery was high resistance with about 3V across it. Connecting it to a 13.8V supply resulted in no current flow.

I levered the plastic top off and exposed the rubber cell caps and removed them.

Then I dripped distilled water from a syringe into to the cells until the level reached the top. Some cells took only a few drops, others half a (20ml)l syringe. I left for an hour or two and came back, some more water had been soaked up so I topped up again.

Measured the voltage, 3V, would not take a charge.

Left it for another hour or two. Took out a 30V power supply and multimeter to measure current, at 30V there was about 10mA current flow. Not looking good. Left it on for 30-60m and noticed the current was very slowly rising. After some more time it got to about 50mA.

Dug out a 50V power supply and set it to constant current mode at 100mA and connected it to the battery. 35-40V was required to get 100mA.

Dripped more water into the cells. Suddenly one or two cells that didn't take any water before would now take loads. Topped them up. Left on charge at 100mA, topping up cells every 15m or so. The voltage started to drop slowly, when it got to about 25V I readjusted for 200mA charge rate.

The voltage continued to drop and I continued to top up the cells.

After a fair while it was at 15V. It's taking a charge at a sensible voltage!

Readjusted for 300mA charge rate. One cell would take more water again.

Now it's charging at 300mA at 14.1V, 14V now 13.9V. Now that seems promising!

It dropped to 13.8V at 300mA after about 1.5hr. So I decided to go for 400mA charge rate. Voltage is now 14.8V, the top end of the charge voltage written on the side of the battery. I wonder if it will continue to fall (i.e. battery recovering) or start to rise (no capacity)...

One cell took a few more drops.

Its fallen to 14.4V at 400mA, that's good. 400mA is probably around the upper charge limit for the battery?

Now its fallen to 13.9V at 400mA charge rate. Good!
The cells took a few more drops. I think I will leave them now, hopefully the excess will be absorbed in time.

13.7V, 400mA. Glued top back on with two blobs of contact adhesive.

Kids bedtime now, so swapped over to constant voltage charging 13.7V